Electrophotographic dry developers are divided into one-component developers comprising a toner itself containing a binder resin having dispersed therein a colorant and two-component developers comprising a toner and a carrier. In carrying out electrophotographic copying using either type of the developer, an electrostatic latent image formed on a photoreceptor, etc. is visualized with the developer to form a toner image, which is then transferred to a transfer material, such as paper or a sheet and fixed thereon by heat, a solvent, pressure, etc. to furnish a permanent toner image. Thereafter, the photoreceptor is cleaned to remove any remaining toner.
Accordingly, a dry developer is required to satisfy various conditions in each of these copying steps, particularly in the development step or cleaning step. For example, a toner should act as independent particles but not in the form of agglomerates. To this effect, it is required that the toner should have sufficient fluidity and that the flow characteristics or electrical characteristics of the toner should not be subject to variation with time or change in environmental conditions such as temperature and humidity.
In addition, the toner in a two-component developer is required to cause no filming phenomenon, i.e., caking of a toner, on the surface of carrier particles.
After transfer of the toner image to a transfer material, the toner remaining on a photoreceptor should easily be released therefrom by cleaning (cleanability). Where a cleaning member, such as a blade or a web, is used in the cleaning mechanism, the toner should not scratch a photoreceptor on being cleaned off with such a member. These properties of a toner in a cleaning step will hereinafter be inclusively referred to as cleaning properties or cleanability.
For the purpose of improving fluidity, cleaning properties and durability of a dry developer, it has been proposed to add to a one-component or two-component developer various external additives, such as inorganic powders (e.g., silica) and organic powders (e.g., fatty acids and derivatives thereof), and fluorine-containing resin powders.
Of the additives proposed to date, inorganic powders, such as silica, titania, and alumina, considerably improve fluidity of dry developers. However, because of their hardness, they are liable to make recesses or scratches on the surface of a photoreceptor. It follows that toner particles cake on the scratched part of the photoreceptor.
Further, regenerated paper has been steadily extending its use with the aim of resources-saving. In general, regenerated paper produces much paper dust, and the paper dust tends to enter the gap between a photoreceptor and a cleaning blade, causing cleaning defects, such as black streaks.
In order to overcome these problems, external addition of a fatty acid metal salt (as described in JP-B-54-16219 and JP-A-60-198556) or a wax (as described in JP-A-61-231562 and JP-A-61-231563) as a lubricant has been proposed. However, any of these external additives disclosed has a large particle size of from 3 to 20 .mu.m so that it should be added in a considerable amount to be made efficient use of. Besides, although these lubricants are effective in the initial stage, they themselves undergo filming, failing to form a uniform lubricating film, eventually causing image defects, such as white spots and blurs. (The term "JP-A" as used herein means an "unexamined published Japanese patent application", and the term "JP-B" used herein means an "examined Japanese patent publication".)
In particular, where an organic photoreceptor in belt form is cleaned with a rubber blade, a brush, etc. in a high-speed copying machine, cleanability of the organic belt photoreceptor is very unstable due to its distortion or sag unlike a drum photoreceptor. Therefore, cleaning of an organic belt photoreceptor must be carried out under a high load of a blade upon the photoreceptor. Further, the state-of-the-art belt photoreceptors have seams, at which a blade chatters or a blade is scratched to cause poor cleaning. Addition of the above-mentioned lubricating additives to the toner has been examined for applicability to such a belt photoreceptor system. It was revealed as a result that the particles added are easily deformed under strong shearing in every case. That is, the additive is effective in the initial stage under a high load but undergoes filming itself before long to cause white spots, blurs, etc.
JP-B-63-39904 discloses a process comprising treating inorganic compound fine particles with a fatty acid or a fatty acid metal salt. According to this process, the effect on cleaning properties is obtained in the initial stage but eventually weakened because the lipophilic inorganic fine particles are gradually buried in toner particles due to stress by agitation and the like. Further, the treatment for imparting lipophilic nature induces agglomeration of the inorganic fine particles. Although the agglomeration gives rise to no problem in the initial stage, the agglomerated particles will be accumulated in a developing machine or be deposited on the inner wall of a developing machine without being consumed. On exceeding a certain level of accumulation, the agglomerated particles appear on the background of copies as coarse grain fog.
It has been suggested to externally add hydrophobic hard fine particles to a toner so that a photoreceptor is abraded by the hard fine particles to prevent toner filming as disclosed in JP-A-2-89064. While effective to prevent filming, the hard particles added wear the surface of a photoreceptor, resulting in a serious reduction in durability of the photoreceptor. The hard fine particles also wear a cleaning blade to reduce the durability of the blade.
In order to meet the increasing demand for low potential and high development performance, several techniques for increasing mobility of a developer have been proposed. Among them is the use of a developer composed of a toner and a low-specific gravity, dispersion type carrier essentially comprising a resin and a magnetic powder. Since such a dispersion type carrier has a low true specific gravity and high insulating properties and can be regulated to have a small size with ease, a more dense and more uniform magnetic brush can be formed than in using a conventional coated type carrier, iron powder, etc. to afford images with improved quality, i.e., satisfactory density reproduction and freedom from noises such as streaks due to the magnetic brush. The developer using such a dispersion type carrier with high developing ability is effective particularly in a low-potential development system. On the other hand, however, due to its own characteristics of low specific gravity, high insulating properties, and small particle size, the dispersion type carrier is carried with a toner for development, and the carrier or a magnetic powder, etc. released from the carrier is apt to adhere on the surface of a photoreceptor. In addition, hard as it is, the magnetic powder tends to scratch the photoreceptor on cleaning, leading to image defects, such as white or black streaks, white or black spots, and the like.
With respect to the transfer step, it is important to generate a uniform electric field in the vicinity of a transfer material. For generation of an electric field, a corotron system has been widely employed for its simple mechanism and low cost. However, a corotron system involves a problem of ozone generation at the time of discharge. Ozone is now under strict control because of its harm to human bodies and, in addition, it contaminates a photoreceptor to cause image defects. Besides the problem of ozone, the corotron system has various disadvantages, such as a need of a high voltage source and a need of maintenance including periodical cleaning for removing deposited substances (e.g., a toner, silicone oil) and discharge products, and renewal of parts in case of burnout. Hence, a transfer system utilizing a bias roll system has been studied in expectation of no ozone generation, no need of maintenance and reduction in the requisite voltage.
In the bias roll transfer system, an electric field for transfer is formed through contact between a transfer material and a bias roll. Accordingly, the two members should be in contact with each other at a linear pressure of at least 5 g/cm for achieving transfer. However, the pressure exerted between a transfer material and a bias roll is ought to be imposed onto the transfer material, the photoreceptor, and the toner image on the photoreceptor. As a result, the toner particles on the photoreceptor tend to undergo agglomeration among themselves or deposition or caking onto the photoreceptor. It follows that transfer of the toner image to the transfer material is inhibited or is not achieved to cause image omissions.
In general development, a toner layer is thicker in the central portion of an image in case of a line image, or in the edge portion of an image in case of a solid image. Therefore, the image omission is liable to occur mostly in the center of a line image and in the edge and its vicinities of a solid image. The occurrence of image omissions is also influenced by the thickness or surface properties of a transfer material. Where a transfer material is thick, the pressure applied to a toner image on a photoreceptor increases so that the toner image is apt to undergo agglomeration or deposition. Where a transfer material has high surface smoothness, for example, in the case of an OHP sheet, the adhesion between toner particles and the transfer material becomes small, easily resulting in an image omission.
In order to increase transfer efficiency, it has been suggested to add various external additives to toner particles thereby to reduce the adhesion among toner particles or between toner particles and a photoreceptor. For example, external addition of inorganic compounds, such as silica, titania, and alumina, has been proposed as described in JP-A-59-226355, JP-A-61-23160, JP-A-63-118757, JP-A-2-1870, and JP-A-2-90175. As long as these techniques are applied to a non-contact transfer system, effects are somewhat expected by specifying the particle size of the additive and by providing adequate air gap between toner particles and a photoreceptor. However, where a transfer material is brought into contact with a bias roll, the inorganic compound, e.g., silica, titania or alumina, is buried in toner particles under pressure because of its hardness, resulting in a failure of performing the function as a transfer improving agent. Further, if part of a toner image remains on a photoreceptor as a result of an image omission, the buried inorganic fine powder also remains thereon to cause recesses or scratches on the photoreceptor.
JP-A-63-279264 teaches external addition of a mixed fine powder of a fatty acid metal salt and a resin aiming at stable supply of an additive while excluding high stress which might be imposed on the additive. This technique, when applied to the bias roll transfer system, is effective to decrease the frequency of image omissions to some extent by virtue of the external addition of a lubricating additive, but the effect reached is still insufficient. In addition, there arises a problem that the resin component in the mixed fine powder tends to be deposited on a photoreceptor.
JP-A-3-121462 suggests external addition of a silicone oil- or silicone varnish-treated fine powder so as to suppress the occurrence of image omissions in a bias roll transfer system. Although this technique produces effects in the initial stage, image omissions still take place in the course of long-term use particularly in copying on ordinary paper under a high temperature and high humidity condition or on OHP sheets under a low temperature and low humidity condition. Further, when stored in a low temperature and low humidity condition for a long period of time, the toner tends to become excessively chargeable, which will develop into a practical disadvantage of insufficient development, resulting in a reduction in image density. Furthermore, when the toner containing the silicone oil- or silicone varnish-treated fine powder is used in a high temperature and high humidity condition for a long period of time, cases are sometimes met in which a photoreceptor is scratched to cause black spots.